Purple pigments based on ceramic materials and colloidal gold may be obtained in various manners and have long been used for producing ceramic decoration and for pigmenting plastics, lacquers, cosmetics, and as glass colorants and decorative colorants.
Until recently, the production of purple pigments comprised several processing stages: (a) precipitation of Cassius gold purple in gel form (colloidal gold adsorbed on tin(IV) oxide hydrate) from an aqueous gold salt solution by means of a tin(II) salt, (b) mixing the moist gold purple with finely ground glass, (c) presintering the mixture at 600 to 800.degree. C., wherein the gold particles are at least partially enclosed in glass and (d) finely grinding the sintered material and, where necessary, establishing the required hue by adding silver compounds and/or other fluxes. Serious disadvantages of this process are that separation of the gold purple in gel form from the aqueous solution of the precursor (a) is very difficult, the hue of the finished pigment is affected in a poorly reproducible manner by the ageing of the gold purple (for example a brown coloration in the event of partial drying) and grinding stages which reduce the space/time yield and an energy-intensive sintering stage are also required. The pigments produced in this manner have furthermore been found not to be sufficiently stable in color at relatively high firing temperatures (of around 1200.degree. C.), i.e. they fade under such firing conditions.
There has been no lack of attempts to improve the process outlined above:
More readily filterable coprecipitates of colloidal gold and the oxide hydrates may be obtained by coprecipitating a gold purple in the presence of oxide hydrate-forming substances, such as Al and Sn salts, wherein the hue of the coprecipitates is simultaneously more readily reproducible and more resistant to ageing--reference is made, by way of example, to RO patent 64442 B (Chemical Abstracts 98 (26): 22 07 40e). However, the remaining processing stages for production of the purple pigments correspond to the above-mentioned stages (b), (c) and (d), such that the overall process still demands considerable effort. According to DD-PS 143 423, the gold purple may also be precipitated in the presence of inert material, for example kaolin, feldspar or pegmatite. The purple, moist precipitate is then homogeneously mixed with a glass flux and, in order to control hue, additionally with a silver carbonate, wet ground, sintered at 650 to 680.degree. C. and ground again. This process too is associated with the above-mentioned disadvantages, in particular with inadequate color stability at a relatively high firing temperature.
Purple pearlescent pigments based on lamellar particles coated with metal oxides, in particular mica coated with TiO.sub.2, having an outer layer of tin oxide accommodating colloidal gold, are known c.f. DE-OS 37 31 174. The coating containing gold is here always located on a metal oxide layer, which must first be applied onto a lamellar support. In the process according to DE-OS 37 31 174, a metal oxide or metal oxide hydrate layer, in particular a layer of TiO.sub.2 or titanium dioxide hydrate, is first precipitated onto lamellar substrates, in particular mica particles, by hydrolyzing metal salts, then tin dioxide or tin dioxide hydrate and elemental gold in colloidal form are simultaneously precipitated in the aqueous phase by hydrolyzing a tin(IV) salt in the presence of a gold salt and a reducing agent, in particular a tin(II) salt. The precipitation stages are followed by a calcination stage at 700 to 1100.degree. C. The production of such pigments is very complicated and thus expensive.
DE-OS 41 06 520 (U.S. Pat. No. 5,252,522) discloses purple pigments based on glass frits having an average particle diameter in the range from 0.5 to 50 .mu.m and a coating containing 0.05 to 5 wt. % of colloidal gold and optionally other hue-modifying metals. The coating is provided by reducing a water-soluble gold compound in an aqueous suspension in the presence of a glass frit by means of a reducing agent, such as glucose. The resultant pigments exhibit unsatisfactory color stability during high temperature firing, i.e. they fade as the firing temperature increases.
According to DE-OS 44 11 104 (U.S. Pat. No. 5,707,436), purple pigments may also be produced by mixing and/or grinding a support material and a gold compound in dry form or by bringing an aqueous solution or suspension of a gold compound and a support material into contact, wherein a precipitating agent for a soluble gold compound, such as ammonia, may be present, and then heat treating the mixture at a temperature above the decomposition temperature of the gold compounds, but below the sintering temperature of the support material, wherein the gold compound is converted into colloidal gold. According to DE-OS 44 11 103 (U.S. Pat. No. 5,589,273), purple decoration may also be produced using a pigment precursor which has been produced as above but not yet heat treated and is thus substantially colorless. In the pigments or pigment precursors described in the two latter-stated documents, the chromophoric metal colloid or the noble metal compound respectively coat the surface of the material without additional oxide hydrate. At a stoving temperature of above 1000.degree. C., such pigments or pigment precursors exhibit inadequate color brightness and reduced color depth as the firing temperature rises.
U.S. Pat. No. 4,839,327 discloses products containing gold which are suitable as catalysts and consist of a finely divided substrate, such as a metal oxide or mixed oxide, having ultra-fine gold particles of a particle diameter of less than 500 nm located thereon. The production process comprises firstly a precipitation of gold hydroxide from a water soluble gold compound onto the substrate in the aqueous phase followed by conversion of the gold hydroxide into colloidal gold by treatment with a reducing agent or by heat treatment. This document mentions neither the color of the product nor the use thereof as a pigment. The thermal stability of the products obtained using this process does not correspond to that required for high temperature firing, i.e. in particular in the range from above 1100 to 1300.degree. C.
Finally, pigments based on glaze-stable compounds, such as ZrSiO.sub.4, SnO.sub.2 or Al.sub.2 O.sub.3, and colloidal gold are known from GB patent 1 436 060. Production proceeds by means of a solid-state reaction. The pigments contain no oxide hydrates and the color has poor brightness and has a dirty appearance.